[RIP]

Worst case you may have cracked the head when the car overheated. Since you aren't saying you have any other driveability issues like slow acceleration, stalling, or misfires I would put a cracked head down on the list. Cost would be around $1000.

Next would be a blown head gasket. It's likely you would see some of the above issues but not necessarily. White vapor out the tailpipe is a sign. Oil in the coolant is a sign. Cost would be around $300 if the head is in good shape.

A leaking freeze plug in the engine block would give you nothing more than what you say you are seeing ie, a leak puddling under the engine and lowering coolant levels. Cost around $100 - 300 depending on where the plug is.

From there you would go to heater hoses or a leak from an improperly installed radiator hose or thermostat housing or even the radiator. Cost $100for the heater hoses and the rest free under the warranty.

You should be able to trace this leak down yourself but, since you just had the system worked on I would take it back to where the work was done. Would have been better if you took it back right away.

The SE light is telling you there is a fail code set in the ECU (computer). Autozone will connect a DRBII scanner and read the code for you for free. This code could give a clue to your leak. As for the fluid, if it's green, blue or yellow it's normal coolant. If it's orange or maybe purple, it's long life coolant. Any brand will do. Mix it 50/50 with distilled water.